Silicon "micromachining" has been developed as a means for accurately fabricating microscopically-small structures. Such processing involves a selective etching of a silicon substrate and a deposition thereon of thin film layers of various materials, both semiconductor and otherwise. Various sacrificial layers are employed to enable the construction of physically movable parts that exhibit microscopic sizes.
Silicon micromachining has been applied to fabrication of micromachines that include rotary and linear bearings. Such bearings have spawned further development of electrically-driven motors which exhibit a planar geometry and lateral dimensions on the order of 100 microns or so. In addition to micromotors, microactuators utilizing cantilever arrangements have also been developed. Such cantilever arrangements employ superposed layers of different materials, each material exhibiting a different thermal co-efficient of expansion (TCE). When the layers are heated, unequal expansion thereof causes a deflection of the cantilever structure.
Cantilever arms have also been fabricated using piezoelectric films which exhibit a large d.sub.31 characteristic. If such a piezoelectric film is sandwiched between a pair of electrodes and is coupled in a cantilever fashion to an electrical contact, application of a voltage across the electrodes causes a flexure of the piezoelectric film and a corresponding movement of the cantilever arm.
In lieu of constructing a cantilever arm having an unattached free end, other prior art has employed a "tied down" cantilever structure to provide a buckling action upon actuation by either a piezoelectric force or a thermally actuated differential expansion action. Piezoelectrically-actuated cantilever devices have also been proposed for a variety of applications, e.g., to control the orientation of micro-mirrors for the purpose of creating an optical switching effect.
The application of electrostatic attraction and repulsion to microactuators has been suggested. In that regard, the following prior art references are relevant: Fedder et al., "Multimode Digital Control of a Suspended Polysilicon Microstructure", Journal of Microelectromechanical Systems, Volume 5, No. 4, December 1996, pages 283-297; Toshiyoshi et al., "An Electrostatically Operated Torsion Mirror for Optical Switching Device", Eighth International Conference on Solid State Sensors and Actuators, Stockholm, Sweden, Jun. 25-29, 1995, pages 297-300; Marxer et al., "MHz Opto-mechanical Modulator", Eighth International Conference on Solid State Sensors and Actuators, Jun. 25-29, 1995, pages 289-292; and U.S. Pat. No. 5,578,976 to Yao; 5,367,136 to Buck and 5,258,591 to Buck.
Physically movable microstructures have been applied to a number of applications. For instance, see Jaecklin et al. "Optical Microshutters and Torsional Micromirrors for Light Modulator Arrays", Proceedings IEEE Microelectromechanical Systems, Fort Lauderdale, Fla., Feb. 7-10, 1993, pages 124-127. Jaecklin et al. disclose a movable shutter arrangement wherein the shutter is positioned on a meander spring which is operated electrostatically to move the shutter in relation to a light-receiving aperture. Comtois et al., "Surface Micromachined Polysilicon Thermal Actuator Arrays and Applications", Solid State Sensor and Actuator Workshop, Hilton Head, South Carolina, Jun. 2-6, 1996, pages 174-177, describe a thermal actuator array wherein a gear structure is rotated to cause the movement of a hinged mirror. Both a thermally actuated gear impeller and a thermally actuated hinged pusher actuator are single layer structures for operating a stepper motor structure.
Shoji et al., "Microflow Devices and Systems", Journal of Micromechanical Microengineering, Volume 4, 1994, pages 157-171, disclose many microvalve, micropump and microflow sensor arrangements that are fabricated using micromachining techniques. Shoji et al. disclose the use of electromagnetic, piezoelectric, pneumatic, shape memory alloy/bias spring, electrostatic, thermopneumatic, electromagnetic and bimetallic techniques for movement of microstructures.
Piezoelectric cantilevers have been utilized in the prior art to construct oscillating structures. Lee et al. "Self-Excited Piezoelectric Cantilever Oscillators", Eighth International Conference on Solid State Sensors and Actuators . . .", Jun. 25-29, 1995, pages 417-420 describe a micromachined, self-excited piezoelectric cantilever which employs a zinc oxide piezoelectric thin film between two aluminum layers on a supporting layer of low-stress silicon nitride. A driving amplifier has sufficient gain to cause the piezoelectric cantilever to oscillate and produce an acoustic output.
Notwithstanding the many actuation mechanisms described in the above-noted prior art, there remains a need for actuation structures which enable a wide range of movement of the actuated device. Further, there is a need for actuation structures provide wide movement angles and large actuation extents.
Accordingly, it is an object of this invention to provide an improved actuator for a microstructure which exhibits multidimensional actuation capability.
It is another object of this invention to provide an improved physical actuator for a microstructure, wherein complex movement of the microstructure is possible.
It is yet another object of this invention to provide an improved physical actuator of a microstructure wherein complex movements of the microstructure can be achieved through either piezoelectric or thermal actuation mechanisms.